1. Field of the Invention
This invention relates to a continuous drilling fluid density measurement system and apparatus for use in a drilling fluid injection system in a subterranean oil or gas well.
2. Description of the Prior Art
The utilization of air bubble probes for monitoring the density of fluids is a technique that has long been practiced. Two such probes are employed and are inserted in the fluid at two different depths so that a pressure opposing the flow of bubbles from each of the two probes is produced on the probes which is proportional to the density of the fluid. Air bubbles are then forced through the probes and out the bottom ends thereof at a constant rate, and the pressure required to produce such constant flow through each of the two tubes is measured. The differential in such pressures is multiplied by a scaling factor to yield the density of the fluid.
This technique is quite satisfactory for use with clean process fluids which do not collect or congeal around the passageway through which the air bubbles are emitted. A sufficient constriction of the air bubble passageway obviously results in a higher pressure being required to maintain the constant flow of bubbles required for the measuring technique. Thus, the increased pressure would result in an erroneous mud density determination.
By their very nature, drilling fluids or drilling mud differ significantly from clean processed fluids because the drilling fluids are intended to collect or congeal in small passageways, such as the passageways through which air bubbles are emitted in a differential pressure density-measuring device. A conventional drilling fluid or drilling mud contains fine particles of clay in suspension which enter and accumulate in crevices and pores in the well bore during circulation of the drilling fluids as part of a rotary drilling operation. The clay in suspension is intended to gel and eventually close the opening through the wall rocks in the well bore to seal the openings against movement of fluid either from or into the well bore.
Mud weight or density-measuring devices using the technique of measuring the differential pressure between two vertically spaced points in a predetermined volume of fluid had been used in conjunction with drilling operations. One such device periodically collects a mud sample having a specified volume. After the weight of each constant volume sample is determined by the differential pressure method, the fluid sample is completely flushed out of the test container before a new, fresh sample is collected through the use of a vacuum. The flushing action permits the instrument to accurately handle foamy mud or mud containing lost circulation materials. This mud-weighing system involves the intermittent sampling and weighing of the drilling fluid, and mud does not stand stagnant in the weighing unit resulting in a false weight recording. See Rogers, Composition and Properties of Oil Well Drilling Fluids; Gulf Publishing Company, 1963, pp. 65-67.
Drilling fluids or drilling mud is circulated through the well bore during a rotary drilling operation. This drilling fluid is used to carry drill cuttings from the bottom of the well to the surface, to lubricate the rotating drill pipe, and to close the pores of formations yielding high-pressure gas or water. Drilling fluid is also used to seal permeable low-pressure formations, fissures, or crevices through which the drilling fluid might otherwise circulate.
Drilling fluid must also have a sufficient density to provide hydrostatic pressure to prevent high-pressure gas, oil, or water from entering the well in a sufficient quantity and rate to cause a blowout. If a high-pressure gas is encountered during drilling, the density of the "gas-cut" drilling fluid, containing entrained gas bubbles is reduced, perhaps to such an extent as to permit a sudden flow of high-pressure gas to enter the well and violently eject the drilling fluid. The density of the drilling fluid is critical in maintaining an opposing hydrostatic pressure to prevent extraneous fluids or gases from entering the well. For example, drilling fluids weighing 70 or 75 pounds per cu. ft. will ordinarily be sufficient to control formation fluids, but when high-fluid pressures are encountered, heavier drilling fluids must be employed. Hydrostatic pressures of about 0.7 psi per ft. may become necessary to maintain adequate hydrostatic pressure.
The density of the drilling fluid can be significantly reduced when gas is occluded in the drilling fluid. When sufficient gas is entrained in the fluid to seriously reduce the density, the differential pressure between the formation and the well is increased and additional gas can enter the well. The volume of gas in the material will also expand at lower pressures contributing to a loss in drilling fluid density.
Drilling fluid or drilling mud comprises a special mixture of clay, water, and chemical additives, pumped downhole through the drill pipe and drill bit. The mud cools and lubricates the drill bit and drill pipe and carries rock cuttings to the surface. The mud serves as a plaster to prevent the wall of the borehole from crumbling or collapsing. Drilling mud also provides the weight or hydrostatic head to prevent extraneous fluids, such as natural gas, from entering the well bore to cause a potential blowout. Among the properties of clay-laden fluids that are important in determining their performance as circulating fluids or muds in rotary drilling are: density, viscosity, colloidity, sheer or gel strength, and sand and salt content. The density of a drilling fluid depends upon the amount and specific gravity of the suspended solids therein. Clay-laden fluids of density and viscosity suitable for rotary drilling purposes range in weight from 8.0 to 24 pounds per gallon and have equivalent specific gravities of 0.96 to 2.88. Drilling fluid or mud weight is measured by the pressure developed by the fluid in pounds per square inch per hundred feet of depth.